There are a number of applications for demountable electrical connections between two circuits. At least one of the circuits may be a connector or an integrated circuit die. Often, contacts are closely spaced, so that any demountable connection device must have a high density of electrically isolated conductors. Specific applications for these connection devices include conducting signals to and from an integrated circuit die to test equipment for determining the electrical properties of-the die, and providing Z-axis electrical conductivity for operating a circuit. A Z-axis microconnection device is sometimes referred to as an "anisotropic conductive elastomer."
One known method for providing small scale, high density interconnections is to embed plated wires within a silicone rubber sheet. The wires provide electrical conductivity in a single direction, i.e. provide Z-axis conductivity. A benefit to this type of conductor is that the silicone rubber provides a degree of compliance. Load compliance is important, since input/output pads of a die may vary in height and since the passivation layer between adjacent pads is susceptible to damage upon the application of force by the microconnection device. A difficulty with this device is that techniques for embedding wires in the sheet impose a limitation on the center-to-center spacing of connections. The smallest linear pitch of the wires is currently 100 .mu.m.
FIG. 1 shows a microconnection device that utilizes plated through holes 10, rather than wires. The plated through holes are metallic members within a porous insulating sheet 12, such as a polyimide sheet. U.S. Pat. Nos. 5,188,702 and 5,136,359 to Takayama et al. describe anisotropic conductive films of the type shown in FIG. 1. Axially aligned bumps 14 and 16 are formed on the opposed sides of each plated through hole 10. A load force can be applied to the upper bumps 14 to press the lower bumps 16 into contact with input/output pads of an integrated circuit die or other circuit of interest. Signals may be channeled to and from the upper bumps 14 by traces formed on the insulating sheet 12 or by conductive members that are pressed against the upper bumps during the application of the load force. The pitch of the bumps 16 may be significantly less than that of the pitch of wires embedded in a sheet. The bumps 16 may be formed to have diameters of 10 .mu.m and may have a pitch of 15 .mu.m. While this provides a greater density of connections than is available with devices having embedded wires, the close spacing limits the load compliance of the device.
As previously noted, one specific application for microconnection devices is testing integrated circuit die to determine electrical properties of the die. Die-level testing is becoming increasingly important as the use of multichip modules (MCM) increases. The process of testing integrated die prior to mounting the die onto a printed circuit assembly or an MCM is known in the industry as "known good die" (KGD) testing. The yields of the various types of die within a particular MCM strongly affects the MCM yield. Die replacement for MCMs is typically cost ineffective. Ideally, die-level testing includes DC tests, functional tests and AC tests, and is carried out by the die suppliers.
KGD testing may be performed using mechanical probes, such as needle-like probes. U.S. Pat. No. 5,207,585 to Byrnes et al. describes a probe for making temporary or permanent interconnections to pads on a semiconductor device. Metallic bumps are pressed against the pads of the semiconductor device to electrically join the device to external circuitry. An article entitled "Evaluation of New Micro-connection System Using Microbumps," by Yamamoto et al., 1993 International Symposium on Microelectronics, SPIE Vol. 2105, pages 370-378 (1993), also describes the use of microbumps for KGD testing.
Conventionally there is a one-to-one correspondence of microbumps to contacts of a circuit of interest. However, techniques for forming plated through holes permit a greater density of microbumps than contacts. FIG. 2 illustrates two microbumps 18 and 20 that contact a first pad 22, with microbumps 24 and 26 contacting a second pad 28. At the opposite side of a flexible membrane 30 are traces 32 and 34. The trace 32 electrically connects the two microbumps 18 and 20 that contact the first pad 22. The trace 34 electrically joins the microbumps 24 and 26 that contact the second pad 28. The traces 32 and 34 extend beyond the connection to the bumps in order to permit contact with external circuitry. These traces may be formed on the upper surface of the flexible membrane 30, or on a carrier 36.
While the known prior art microconnection devices for KGD testing and other applications requiring high density anisotropic conductive pads work reasonably well for intended purposes, concerns regarding reliability exist. Firstly, there remains a possibility that an incorrect circuit-test failure will register when, for example, a compression force is weak. While forming the microbumps on a flexible membrane will provide some vertical compliance, the close spacing of the microbumps significantly limits the amount of flexing that can take place between the microbumps. Another concern is that a false indication of failure may occur if either the microbumps or the contacts of the circuit of interest are stained with a nonconductive material, e.g. oil, or have acquired a layer of oxide. For example, an aluminum input/output pad of a semiconductor die may have a thin layer of aluminum oxide that jeopardizes the electrical connection between the microbumps and the pad.
What is needed is a microconnection device and method that provide a high level of uniformity of compression force. What is further needed is such a device and method that reduce the susceptibility of solder-free microconnections to false indications of circuit discontinuity.